Difference between revisions of "Guide number"

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The best of the three pictures will give the guide number
 
The best of the three pictures will give the guide number
  
test guide number = official guide number * sqrt(ISO speed/speed of official guide number)
+
test guide number = official guide number * sqrt(film speed/speed of official guide number)
  
 
real guide number = aperture * distance between flash and image subject
 
real guide number = aperture * distance between flash and image subject

Revision as of 19:19, 28 May 2006

The guide number is a value indicating the strength of a flashlight. According to Andreas Feininger the guide number helps finding the right aperture for flashlight exposures with the formula


aperture=                guide number            
         distance between flash and image subject
(in meters)

Since different manufacturers give guide numbers for different basic ISO filmspeeds (50 or 100 ASA) or even no guide number for their flashes Feininger suggests the user to find out the guide number himself by making 3 test exposures with the film he wants to use for flash photography. The image subject must have the same distance for each test exposure, distance and aperture must be notified. The aperture for the first test photo must be selected according to the formula when calculated with the official guide number. If the film has another speed than the ISO speed on which the official guide number is based this guidenumber must be recalculated for the other film speed. The further exposures shall be made with aperture below or above the value calculated for the guide number.

The best of the three pictures will give the guide number

test guide number = official guide number * sqrt(film speed/speed of official guide number)

real guide number = aperture * distance between flash and image subject

The effective guide number can be different depending on factors like the subjects light reflection or alternate flash usage ways like indirect flashing or flash reflector adjustment.

Modern all-automatic cameras don't let the users worry about all that stuff. They optimize flash exposures with their through-the-lens metering "on the fly". But even for those cameras the guide number helps to understand the limited distance on which a flash is effective, see formula


maximum distance=    guide number      
                 maximum aperture